drm/fbdev: Select FB_CORE dependency for fbdev on DMA and TTM

Select FB_CORE if GEM's DMA and TTM implementations support fbdev
emulation. Fixes linker errors about missing symbols from the fbdev
subsystem.

Also see [1] for a related SHMEM fix.

Fixes: dadd28d4 ("drm/client: Add client-lib module")
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/series/141411/

 # 1
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241216074450.8590-3-tzimmermann@suse.de